The present invention relates to a cushioning device for an athletic shoe and, more particularly, to such a cushioning device that imparts both stability and cushioning to the midsole of an athletic shoe.
Athletic shoes typically include supporting and cushioning structures to absorb the force of impact associated with running and jumping. The supporting and cushioning structures are often positioned in the rear foot or heel section of the shoe in order to absorb a portion of the shock encountered by the wearer. Such structures may be designed to increase the stability of the shoe by supporting the wearer""s foot against undesirable rotation relative to the leg. Pronation, rotation of the inner, or medial, side of the foot downward relative to the outer, or lateral side, and supination, the opposite rotation of the medial side upward relative to the lateral side, are two types of excessive rotation that can cause foot injury. These rotations are prevented or alleviated by the structures that resist the rotational motion of the foot.
Fluid bladders are one type of structure used to provide cushioning and stabilizing in athletic shoes. Bladders of this type include fluid filled chambers or pockets that are fitted in the midsole of the athletic shoe. The pockets may be pressurized beyond the ambient pressure level, or may be filled with fluid at the ambient pressure. U.S. Pat. No. 5,575,088 discloses a fluid bladder imparting cushioning to a heel section of a shoe. The bladder includes individual, concentric chambers that are connected so as to allow fluid to be communicated between the chambers. The concentric chambers are ring shaped with the inner ring having a lower height than the outer ring. The arrangement forms a cradle for the heel, providing support and stabilization therefor. The pressure within the chambers of the bladder is uniform because fluid pressure equilibrates between the ring sections, which are in fluid communication with one another. U.S. Pat. No. 5,353,459 discloses a bladder in which separate chambers are maintained at different pressures through the use of distinct interconnecting tubes.
The present invention improves upon the cushioning and stabilizing characteristics of existing bladder arrangements. The bladder arrangement of the present invention includes a plurality of separate, fluid filled chambers positioned around a perimeter. The chambers are positioned so that an area having the greatest volume is located towards the outer side thereof. The individual chambers may be connected to one another by connective elements, but are not in fluid communication with one another. Under angled impact by a wearer""s heel, the fluid filled chambers absorb the force of impact. The higher volume section of one or more of the chambers is subjected to compressive forces first. The loading of the higher volume section forces pressure into the lower volume section(s) thereby providing an increasingly greater resistance as the higher volume section is further compressed. The building of resistance provides a gradual reduction of the impact forces encountered by the wearer as the higher volume section(s) deflects.
In a preferred embodiment of the invention, the bladder arrangement includes a rear central chamber, a first side (medial) chamber and a second side (lateral) chamber. The fluid pressure of the medial chamber is maintained at a higher pressure than the fluid pressure in the other chambers in the arrangement. The higher fluid pressure provides increased stiffness of the medial chamber and provides added protection against pronation.
The fluid bladder is preferably encapsulated by a cushioning material such as polyurethane foam. The encapsulated fluid filled bladder arrangement, that comprises the cushioning device, is positioned in the space in the rear foot region of the shoe midsole.